refactor backend socket/connection handling

refactor backend socket/connection handling

We now take steps to ensure the socket we wrote to is always the same as
the socket we'll read from.  And if a read ever fails we'll close the
socket to avoid having responses.  We've also switched from Socket#send
to IO#write as IO#write guarantees write-in-full behavior as long as the
socket is alive (MogileFS queries are small)

The public MogileFS::Backend#shutdown method is now thread-safe,
too.